This invention relates to a radiation curable pressure-sensitive adhesive composition which has high cohesion and is excellent in adhesiveness.
For allowing pressure-sensitive adhesive tapes to exhibit their adhesion characteristics such as the desired cohesion, peel strength, etc., it has heretofore been necessary to use a polymer having a relatively high molecular weight and viscosity (e.g., natural rubber, synthetic rubber, etc.). Therefore, in coating, the polymer should be applied to a substrate in the form of a solution in an organic solvent, and since a large amount of the solvent used is allowed to evaporate, a long period of time is required for a drying step after the coating. Moreover, solvents used for the above-mentioned purpose are volatile and flammable and many of them have harmful effects on human body; therefore their empolyment often causes a fire or various environmental pollution problems. Further, a solvent-recovering apparatus which is said to be provided inevitably from the viewpoint of economics and preventing the above-mentioned environmental pollution is generally expensive and requires a wide space for its provision. Accordingly, in recent years, making a pressure-sensitive adhesive solventless has come to be noted from the viewpoint of the so-called saving-resources, saving-energy and preventing-environmental pollution. As counterplans for making a non-solvent, there are adhesives of emulsion type, hot melt type and otherwise, but in purticular, radiation curable pressure-sensitive adhesives using a liquid oligomer having one or more unsaturated double bonds in its molecule are highlighted. Reasons for this are as follows. The radiation curable pressure-sensitive adhesives can be produced in the form of the so-called solventless adhesives containing, in principle, no organic solvent which is a defect of the solution type pressure-sensitive adhesives described above, or only a small amount of an organic solvent, if any. And they are characterized in that (1) their curing (polymerization) reaction is rapid because radiation which is an actinic energy ray is used, (2) the usable life can be controlled freely because the curing reaction proceeds only during irradiation, and (3) for their production, no large dryer is needed.
However, even the radiation curable pressure-sensitive adhesives have the following defects. In general, the polymerization (curing) reaction proceeds to unsaturated double bonds by a radical produced after irradiation, but the reaction rate is so rapid to cause excessive crosslinking. As a result, the glass transition temperature is raised. Therefore, the resulting film is brittle, and in an extreme case, serious polymerization shrinkage occurs, so that the film is cracked. When the film characteristics in this condition are evaluated, the film has high tensile strength but shows almost no elongation and no rubber-like properties. On the other hand, when the absolute amount of unsaturated double bonds in the molecule is decreased in order to inhibit the excessive crosslinking, the reaction proceeds locally, so that no sufficient cohesion can be obtained as a whole.
As described above, it has been difficult to control the curing reaction appropriately and allow to retain their adhesion characteristics as pressure-sensitive adhesives.